Engine will not start wilst cranking with the key.

Fault code present that will not clear (P251A) which is for the power take off switch open circuit??
Van starts with a squirt of eaystart and runs sweat afterwards. Will not restart when switched off.
Fault codes read with Snap On scanner, Sykes SPi, and DEC.
When cranking with DEC connected there is no syncronisation of CAS and CPS but once running it tells me they are, CPS test ok and a new one has been tried.
Cylinder head temperture sensor reads -18 deg, but I have noticed this on other Transits, Mondeos and Jaguars that I have tested, so I believe this is not relevant.
Many thanks for reading

Ivor Jones